The present invention relates to stationary bicycle and relates more particularly to a structure of magnetic retarding field for retarding the rotary motion of stationary bicycle's flywheel wherein the retarding force from the magnetic field onto the flywheel can be conveniently adjusted by changing the depth in which the flywheel cuts and, wherein the magnets which are disposed at the two opposite sides of the flywheel to produce a magnetic retarding field therebetween are constantly maintained at a fixed range from the flywheel.
Various structures of magnetic retarding field for retarding stationary bicycle's flywheel have been disclosed in U.S. Pat. Nos. 4,186,320 issued to Hillman; 4,752,066 issued to Housayama; 4,822,032 issued to Whitmore, et al.; 3,831,942 issued to Del Mar. In these disclosures, two sets of magnets are separately mounted at two opposite sides relative to the flywheel to produce a retarding field therebetween, wherein at least one set of the magnets is immovably fixed in position. In order to achieve best retarding field, magnets must be mounted at the two opposite sides of the flywheel with less space left therebetween (less than 2 m/m is allowed between each magnet and the flywheel). However, the flywheel may be biased to contact either magnet due to impact accident during transportation or bad calibration. Further strong magnetic attraction between the two opposite sets of magnets may force each magnet mounting rod to deform, causing the magnets to contact the flywheel. Although in the disclosure of U.S. Pat. No. 4,822,832 an U-shaped magnet is used to generate a magnetic field for retarding the flywheel, it still can not eliminate the aforesaid problem because it is vertically movably mounted on the stationary bicycle frame and separated from the flywheel.